This invention relates to a water-cooled wall and roof panel for installation in an electric-arc furnace used for melting metal material and refining molten metal.
In electric-arc furnaces used to melt metal materials and refine molten metal up to the mid-1970s, the furnace body was lined with refractory at the inside of shell and the roof used arch-like refractory at the inside of a metal frame called a furnace roof frame. From around the latter half of the 1970s, however, the need to increase productivity led to a rapid increase in electric-arc furnace size. Since electric-arc furnace power consumption rose in proportion, wear and tear on the refractory used in the furnace body and roof increased markedly. The result was higher refractory-related costs and more downtime for refractory repair.
One attempt made to overcome these problems was aimed at extending the refractory service life while minimizing a decrease in electric-arc furnace heat efficiency by using a furnace body cooler constituted as an structure of one or more cooling water pipes and bricks embedded within cast iron or a copper casting (Japanese Unexamined Published Utility Model Application 49-118635). However, in the cooling structure using cast iron, the iron casting of the cooler proper reaches a temperature of 1000xc2x0 C. at the surface on the furnace interior side. During use for several hundred to one thousand charges, therefore, the casting experiences cracking caused by thermal stress and becomes brittle owing to change in texture. As the cracking and embrittlement proceed, the casting undergoes wear and the bricks within the casting drop out. When the cracks occurring in the casting surface propagate as far as the cooling water pipe(s), water leakage occurs. In the cooling structure using cast copper, wear and cracking do not arise quickly because the copper casting has higher ductility than the iron casting and does not experience texture change. Still, since the bricks are embedded in the panel on the furnace interior side and the pipe or pipes are present behind the bricks, the ends of the bricks on the furnace interior side reach a high temperature that causes them to wear rapidly. The panel proper also has greater weight owing to its larger thickness. The material cost is therefore higher than when cast iron is used, especially in the case of a very large copper casting.
On the other hand, Japanese Unexamined Published Utility Model Application 56-29798 teaches a method for overcoming the foregoing problems by casting a low-melting-point metal such as copper or aluminum around a cooling water pipe so as to branch radially, thereby enhancing cooling capability and preventing propagation of cracks occurring at the casting surface. Calculations show that this method should hold the temperature of the casting of the cooler proper on the furnace interior side to around 500xc2x0 C. In fact, however, when the cooler is installed at a high thermal load location where slag does not adhere or barely adheres to the casting surface, the surface temperature reaches 1000xc2x0 C. or higher. Because of this, the problem of texture change and cracking of the casting cannot be overcome. This method also increases cost because complicated fabrication steps are required in order to cast the low-melting-point metal, which has different properties from the cooler proper, around the cooling water pipe.
Because of the texture change and cracking of the casting, along with other problems, the cast cooler of this structure has not come into general use. The most commonly used structure used today is the water-cooled panel used in a furnace having no refractory at its inner surface and constituted as a cooler of welded steel plate structure, steel pipework structure, copper casting structure or welded copper plate structure. The water-cooled panel is helping to reduce refractory wear also in large-size, high-power electric-arc furnaces. (See, for example Japanese Unexamined Published Patent Applications 51-97506, 56-66680 and 56-45800.)
Various ways have also been suggested for increasing the durability of the electric-arc furnace roof. A vertical sectional view of a conventional electric-arc furnace is shown in FIG. 13. The top of the shell 21 of the electric-arc furnace is closed by an openable roof 23 made of refractory and formed with electrode insertion holes 16 for passage of electrodes 22. During operation, the refractory roof 23 incurs fusion damage under high-temperature heating and has to be replaced. This increases cost. In response to this, Japanese Unexamined Published Patent Application 53-107729 teaches the furnace roof shown in vertical section in FIG. 14. All of the furnace roof, except for the inverted cone portion formed with the electrode insertion holes 16 for passage of the electrodes 22, is made of steel plate and the interior of this portion is formed with helical passages 24 to constitute a water-jacket roof 25. The inner surface of water-jacket roof 25 is formed with a metal film 26 of high thermal conductivity and capable of reflecting radiant heat. This structure prolongs the service life of the furnace roof.
Still, owing to the occurrence of cracks with continuing operation of the electric-arc furnace, this water-jacket type furnace roof made of steel plate frequently experiences water leakage from the water jacket. Moreover, in an electric-arc furnace whose wall and roof are formed with water jackets made of steel plate, the amount of heat lost to water cooling accounts for about 10% of the total energy required by the electric-arc furnace. About half of the lost heat is carried away by the roof cooling water. Also in the electric-arc furnace roof, therefore, there is a need to reduce the amount of heat lost to the cooling water without increasing wear of the refractory.
Japanese Unexamined Published Patent Application 50-142709 teaches a roof for an electric-arc furnace that uses an appropriate number of coolers composed of one or more cooling water pipes and bricks embedded in cast iron, cast copper or other such casting. This furnace roof reduces the amount of heat lost to the cooling water. However, the furnace roof of this structure has the same problems as pointed out regarding the furnace body cooler describe earlier. Specifically, the casting of the cooler proper reaches a temperature of 1,000xc2x0 C. at the surface on the furnace interior side. During use for several hundred to one thousand charges, therefore, the casting experiences cracking caused by thermal stress and becomes brittle owing to change in texture. As the cracking and embrittlement proceed, the casting undergoes wear and the bricks within the casting wear and drop out. When the cracks occurring in the casting surface propagate as far as the cooling water pipe(s), water leakage occurs.
Therefore, like the furnace body cooler, the furnace roof cooler is also susceptible to cracking of the steel plate and the steel pipework portion as well as to the water leakage this causes. Despite such shortcomings, coolers of the welded plate structure and steel pipework structure, known as water-cooled panels, are in general use.
The technologies described in the foregoing attempt to reduce wear of the electric-arc furnace refractory, lower cost and decease downtime for refractory repair by equipping the furnace interior with water-cooled panels which, being of welded steel plate structure, steel pipework structure, copper casting structure or welded copper plate structure, have no refractory on the furnace interior side. Owing to the absence of refractory on the furnace interior side, however, the water-cooled panel must be supplied with a large amount of cooling water to protect the panel proper. Problems therefore persist regarding heat loss to the cooling water and the need for a high-power pump for supplying the cooling water. Against the backdrop of intensifying calls for more efficient energy utilization in order to reduce emission of carbon dioxidexe2x80x94a greenhouse gas that promotes global warmingxe2x80x94a need therefore exists for a water-cooled panel for electric-arc furnaces that can lower the amount of heat lost to the cooling water and reduce the amount of power consumed by the pump used to supply the cooling water, without increasing refractory wear.
The conventional furnace body cooler composed of one or more cooling water pipes and bricks integrally embedded in an iron casting (Japanese Unexamined Published Patent Application 49-118635) experiences cracking caused by thermal stress and becomes brittle owing to a change in texture. As the cracking and embrittlement proceed, the casting undergoes wear and the bricks within the casting drop out. In the cooling structure using cast copper, although no cracking arises because of thermal stress and no embrittlement is caused by change in the casting structure, the ends of the bricks on the furnace interior side wear rapidly because they are not cooled.
Although refractory brick dropout can be completely prevented by maintaining the casting in sound condition, the surface temperature of the refractory brick rises above 1,000xc2x0 C. even after the cooling capability has been upgraded. Moreover, it has not been possible to avoid gradual, progressive oxidative wear of the refractory brick surfaces in a high temperature atmosphere and/or mechanical damage to the refractory bricks under the impact of scrap charged into the electric-arc furnace. Therefore, when brick wear proceeds to the point that the effect of reducing heat loss to the cooling water can no longer be obtained, the water-cooled panel proper has to be removed and replaced. The old water-cooled panel, which cannot be refurbished with new refractory bricks, has to be scrapped. This is another disadvantage.
When the panel is applied to the wall and roof of an electric-arc furnace, slag and other furnace deposits are retained stably on the furnace wall. Loss of heat to the cooling water is therefore lower than in the case of the water jacket type panel. In the case of the furnace roof, however, the slag and other furnace deposits tend to fall into the furnace, making stable retention difficult. This is because the refractory bricks are made smaller in width at their inner ends than at their outer ends and also because of the rectangular shape of the protrusions for stably retaining the slag and other furnace deposits. The slag and other furnace deposits provide a marked heat insulating effect. The reduction of heat loss to the cooling water is therefore less reliable when the panel is applied to the furnace roof than when it is applied to the furnace wall. Moreover, the more frequent exposure of the bricks embedded in the panel to the furnace interior accelerates brick wear.
The present invention was accomplished to overcome the foregoing problems and provides a water-cooled panel for the wall and roof of an electric-arc furnace that reduces heat loss, reduces power needed for cooling water supply, and achieves a service life equal to or longer than a water-cooled panel of welded steel plate structure, steel pipework structure, copper casting structure or welded copper plate structure having no refractory at the furnace inner wall.
The water-cooled panel for the wall and roof of an electric-arc furnace according to this invention is a cast iron, cast steel or copper casting type water-cooled panel integrally fabricated of refractory bricks arrayed on the furnace inner wall in multiple regularly spaced rows to be exposed at the end faces and at least one cooling water pipe installed between the rows of refractory bricks.
In the foregoing structure, the refractory bricks can be embedded with their ends on the furnace interior side projecting from the casting surface, the refractory bricks can be tapered to make the width of their ends on the furnace interior side smaller than the width of their ends on the side opposite the furnace interior side, the refractory bricks can be formed to have rounded corners at their ends on the side opposite the furnace interior side, cushioning material can be disposed between the contacting surfaces of the refractory bricks and the casting, and the casting surface on the furnace interior side can be locally formed with ridges.
In accordance with another feature of the present invention, the water-cooled panel for the wall and roof of an electric-arc furnace wall is a water-cooled panel wherein slits for inserting refractory bricks from the side opposite the furnace interior side are arrayed in multiple regularly spaced rows and at least one cooling water pipe is embedded between the rows of slits, one of the following structures being adopted:
1) The slits for inserting refractory bricks are formed straight to have the same width at the end on the furnace interior side and the end on the side opposite the furnace interior side;
2) The slits are tapered to have smaller width at the end on the furnace interior side than at the end on the side opposite the furnace interior side;
3) The ends of the refractory bricks on the side opposite the furnace interior side are made to project from the casting surface and are secured by metal fasteners provided on the side of the water-cooled panel opposite the furnace interior side;
4) The refractory bricks are secured by multiple recesses formed in projecting portions of the refractory brick on the side opposite the furnace interior side and multiple protrusions formed in refractory brick metal fasteners;
5) The ends of the refractory brick on the furnace interior side are secured to project from the casting surface;
6) Cushioning material is disposed between the refractory bricks and between the contacting surfaces of the refractory bricks and the casting;
7) The casting surface on the furnace interior side is locally formed with ridges.
A water-cooled panel for an electric-arc furnace roof according to the present invention is a panel composed of multiple refractory bricks and one or more cooling pipes for passing cooling water embedded in cast iron, cast steel or copper casting, wherein the refractory bricks project from the cast iron on the furnace interior side, the ends of the refractory bricks projecting on the furnace interior side and the portions thereof embedded in the cast iron are formed in a shape larger than the width of the middle portion, and the surface of the cast iron on the furnace interior side is provided with slag catchers for retaining slag adhering to the furnace roof, water-cooled panels for an electric-arc furnace roof of this structure being contiguously arranged on a frame in ring shape to form an electrode insertion hole at the middle.